We have confirmed that somitogenesis is a multi-stage process, with the initiation of the process being correlated with the acquisition of adhesiveness by the anterior cells of the segmental plate (the presomitic mass). At stage 0 (zero) there is no obvious evidence of eventual segmentation except for the meristic whorls (somitomeres) originally described by Meier (cf. Meier, 1984). These somitomeres can be seen under suitable illumination in isolated, live segmental plates. Stage l is marked by the anterior cells of the segmental plate becoming adhesive and undergoing a process of aggregation that can be described as "compaction". This acquisition of adhesivness is promoted by cellular fibronectin in three different experimental situations: isolated segmental plates, dissociated segmental plates, and intact embryos. Fibronectin derived from plasma seems to have minimal effect upon these processes. Using specific peptides representing the adhesion domain of the fibronectin molecule we have established that a specific peptide sequence (RGDx) perturbs adhesion in the segmental plate cells, but appears not to mimic exactly the action of the intact fibronectin molecule, suggesting that other factors or molecular domains are implicated in the initiation of somitogenesis. We plan to probe the molecular aspects of these interactions by preparing monoclonal and polyclonal antibodies to cellular fibronectin. Additional molecular probes and antibodies are available to us to pursue this problem in more detail. The thrust of this proposal is to investigate in detail the interaction between the segmental plate cells and specific ligands. Concurrent studies will be performed to analyze morphogenetic movements utilizing immunohistology and scanning electronmicroscopy.